Tomographic imaging methods are characterized by enabling the internal structures of an object under examination to be examined without having to carry out operational interventions on said structures. One possible type of tomographic imaging includes recording a number of projections from different angles. A two-dimensional slice picture or a three-dimensional volume picture of the object under examination can be computed from these projections.
Computed tomography is an example of this type of tomographic imaging method. Methods for scanning an object under examination with a CT system are generally known. Typical methods employed in such cases are orbital scans, sequential orbital scans with advance or spiral scans. Other types of scans which are not based on circular movements are also possible, such as scans with linear segments for example. Absorption data of the object under examination is recorded from different recording angles with the aid of at least one X-ray source and at least one detector lying opposite said source and this absorption data or projections collected in this way are computed by means of appropriate reconstruction methods into picture slices through the object under examination.
For the reconstruction of computed-tomographic pictures from X-ray CT datasets of a computed-tomography device (CT device), i.e. from the recorded projections, what is known as a Filtered Back Projection (FBP) is used nowadays as the standard method. After the data has been recorded, a so-called “rebinning” step is executed in which the data generated with the beam spreading out in the form of a fan is rearranged such that it is available in a form such as would occur had the detector been hit by X-rays arriving at the detector in parallel. The data is then transformed into the frequency range. A filtering is undertaken in the frequency range and subsequently the filtered data is back transformed. With the aid of the data sorted out and filtered in this way, a back projection is then carried out onto the individual voxels within the volume of interest.
A disadvantage of this generally-known computation method lies in the fact that with a moving object under examination or an object under examination which moves at least in part, movement imprecision can arise in the picture, since during the period of a scanning process for the data which is needed for a picture a local displacement of the object under examination or of a part of the object under examination can occur, so that the basic data which leads to the picture does not all reflect spatially identical situations of the object under examination. This movement imprecision problem arises particularly acutely during the execution of cardio CT examinations of a patient for whom, as a result of the heart movement, a strong movement imprecision can occur in the area of the heart or for examinations in which relatively rapid changes in the object under examination are to be measured.
The undesired movement artifacts are reduced by the temporal resolution of the CT imaging being increased. There are various methods of doing this. On the one hand it is possible to shorten the rotation time of the gantry. However in this case, mechanical restrictions are soon encountered since the centrifugal force exerted on the components increases quadratically as the rotation time reduces.
On the other hand, as part of the picture reconstruction, by using phase-equivalent angle-complementary data of adjacent heart cycles, the temporal resolution can be improved. However the benefit depends on the ratio of the heart rate to the gantry circulation time and is barely able to be influenced.
Finally two-emitter CT systems have been developed, i.e. CT devices with two X-ray sources and detectors assigned to them. In accordance with the halved measurement time as a result of the presence of two X-ray source-detector systems, these make a doubled temporal resolution possible. The disadvantage in this case is that the costs for the duplicated design of the core components, such as emitter, detector etc., are significant.